In the manufacture of insulated conductors such as those used in communications, metallic stock in the form of rod such as copper rod, for example, typically is reduced in diameter prior to covering it with a plastic insulation. This process is referred to as wire drawing and includes the step of advancing the rod through a plurality of successively smaller die openings to provide a wire which subsequently is insulated with a dielectric material such as a plastic material.
Each time the metallic conductor material is cold worked such as by causing it to be pulled through a die opening, the metallic grain structure is altered. This increases the number of dislocations through which electrons must travel during the flow of current. In other words, the resistivity of the resulting copper wire is increased through cold working.
In order to reduce the effects of cold working, the moving wire is annealed prior to plastic material being extruded thereover. The process of annealing is used to heat the moving wire for purposes of recovery, recrystallization and grain growth when sufficient thermal energy is available for grain growth. Annealing decreases the number of dislocations and consequently improves electron flow. Accordingly, the resistivity of the wire is decreased and its conductivity is increased.
It follows that the less the resistivity, the less the amount of copper which will be required to meet product specifications. With less copper required to satisfy product requirements, the amount of raw material is reduced, and the final cost of the product will be lowered.
After the wire has been annealed, it may, depending on the desired properties of the final product, be cooled. If it is cooled, then typically it is reheated in order to control more accurately the temperature of the wire as it enters an extruder in a tandem insulating line.
It has been found that copper wire is annealed more efficiently and more consistently on some tandem insulating lines than on others. Further, specific tandem lines seem to anneal copper wire better than other lines; that is, on some lines the annealing is more consistent for each successive increment of length of the wire than on others. Although there may be variations in equipment among tandem lines in a single manufacturing plant, there is one common denominator--the annealer.
Variations in the electrical energy imparted to the moving wire are undesirable. If such variations go uncontrolled, either more copper must be used through a larger cross section of the wire or more electrical power is used to compensate for the fluctuations in order to achieve desired properties. What is needed are methods and apparatus for inhibiting variations in the electrical energy imparted to the wire during annealing in order to optimize the amount of copper and electrical power used and to achieve increased conductivity.
It appears that the prior art has not yet addressed this problem. What is needed are methods and apparatus for heating a moving wire in such a manner that the energy imparted to each successive increment of length of the wire is substantially constant. Desirably, such methods can be implemented with a minimum amount of investment and minimal modifications of existing equipment.